The present invention is related to vapor deposition growth of semiconductor materials and to associated apparatus and methods. More specifically, the present invention is related to a wafer processing apparatus and a wafer processing method for reducing reactant memory in the relevant apparatus chambers.
Crystal growth from vapor is employed in semiconductor technology, in particular, for producing epitaxial layers on semiconductor wafers. The term epitaxy typically describes the growth of a monocrystalline layer on the planar boundary surface of a monocrystalline substrate, generally a substrate wafer of a semiconductor material.
Epitaxial growth is often carried out using chemical vapor deposition (CVD) in CVD reactors. In such processes, the semiconductor wafer is first heated and then exposed to a gas mixture, referred to as a process gas. The process gas mixture typically consists of a source gas, a carrier gas, and, where appropriate, a dopant gas. The source gas (or gases) provides the elements that form the desired semiconductor; e.g. trimethyl gallium and ammonia to form gallium nitride. The dopant gases carry (typically as compounds) elements that add p or n-type conductivity to the epitaxial layer; e.g. magnesium to obtain p-type gallium nitride. The source and dopant gases react on or near the hot substrate surface to form the desired epitaxial layer.
In a typical CVD process, reactant gases (often diluted in a carrier gas) at room temperature enter the reaction chamber. The gas mixture is heated as it approaches the deposition surface, heated radiatively, or placed upon a heated substrate. Depending on the process and operating conditions, the reactant gases may undergo homogeneous chemical reactions in the vapor phase before striking the surface. Near the surface thermal, momentum, and chemical concentration boundary layers form as the gas stream heats, slows down due to viscous drag, and the chemical composition changes. Heterogeneous reactions of the source gases or reactive intermediate species (formed from homogeneous pyrolysis) occur at the deposition surface forming the deposited material. Gaseous reaction by-products are then transported out of the reaction chamber.
Because a p-n junction is a fundamental element in many semiconductor devices, epitaxial layers of opposite conductivity type are often grown consecutively to one another on the substrate, typically by changing the composition of the dopant gas at a desired point during the growth process. Similarly, when heterostructures are produced using CVD, the composition of the source gases is similarly changed.
Such changes in source or dopant gas composition can lead to a problem referred to as “reactant memory.” The term “reactant memory” describes the undesired contamination of the process gas with source or dopant compositions or elements that remain in the chamber from previous deposition steps. At elevated temperatures, dopant and source compositions are capable of sticking to the reactor walls and potentially re-evaporating during following epilayer depositions. When, for example, dopants re-evaporate, the possibility exists that the dopants will be included or incorporated in the subsequent epi layers. In such layers the dopants can act as impurities or can change the electronic characteristics of the layers and the subsequent devices. This effect is often more pronounced for aluminum and boron than for nitrogen in SiC epitaxy. The effect is also pronounced for telluriumand zinc in GaAs epitaxy and for magnesium in GaN epitaxy.
Doping control is intricate in the epitaxial growth procedure. The background doping can be limited by using purified gases, and high-grade materials in the critical parts of the reactor. Memory effects from earlier growth steps where dopants have been intentionally introduced are also problematic.
Several attempts have been made to overcome the problems associated with reactant memory. One such attempted solution is site-competition epitaxy. Site-competition epitaxy is based on the competition between, for example, SiC and dopant source gases for the available substitutional lattice sites on the growing SiC crystal surface. In this case, dopant incorporation is controlled by appropriately adjusting the Si:C ratio within the growth reactor to affect the amount of dopant atoms incorporated into these sites, either carbon-lattice sites (C sites) or silicon lattice sites (Si sites), located on the active growth surface of the SiC crystal. This technique has also been utilized for arsenide and phosphide growth. By using site-competition epitaxy, the impurity level of the epilayer can be controlled by adjusting the C:Si ratio, while the n-type dopant nitrogen is increased at a low C:Si ratio. Hence, the C:Si ratio must be chosen to limit the domination dopant to grow low-doped material, while intentionally doped material must be grown under the C:Si ratio most suited for the dopant of choice.
Previous methods for counteracting reactant memory have also included cleaning the reactor after each deposition, baking out the reactor, and burying the dopant by re-coating the reactor walls. Another method for controlling the effect includes etching the reactor walls after each doped layer has been grown, for example using hydrogen or a hydrochloric acid. Combinations of an etch and an active C:Si ratio control have also been utilized to avoid the problems of reactant memory. These solutions, however, suffer from several drawbacks. Each method is time-consuming and reduces output, and adds additional processing steps to the technique. These methods may also result in growth stop effects such as poor adhesion between layers. Moreover, the various proposed solutions to the problem of reactant memory can also be costly additions to production of the desired devices.
Defect control has been considerably improved by optimizing the cleaning procedure before growth, both ex-situ before loading, and in-situ as part of the growth sequence. Reactant memory has not, however, been sufficiently reduced using these techniques to allow for efficient low doping epitaxial growth of multiple layers in some processes. It would therefore be desirable to develop an improved and more efficient technique for epitaxial growth while avoiding defects caused by reactant memory.